Wood-type heads are traditionally made from wood. However, with advances in materials, wood-type golf club heads have been made from various high-performance metals and other materials, such as titanium and fiber-reinforced plastics. Most club heads from fiber resin composite material are compression molded around relatively rigid molding cores. The majority of these finished club heads have a low quality composite, which is the result of difficulties in the process. To obtain a high quality composite in the club head using a rigid molding core, the core must be shaped very specifically and the uncured fiber resin material must be placed very precisely and systematically on the core to ensure that the proper amount of compaction occurs at all locations inside the golf club head.
Very few manufacturers have been willing to spend the money it takes to produce a high-quality composite in a golf club head. Instead, they often employ a process that combines an uncured resin with a heat-activated foaming agent around a foam core. Under low pressure, it expands and ensures almost full contact of the interior foam material to the outer composite material. The composite material in such heads is of a very low quality with high void content, low fiber content, and wrinkling of the reinforcing fibers.
In the few cases where the manufacturer has taken the time and expense to very accurately size and position all the materials going into the club head, other problems were encountered. The fiber resin material must be carefully positioned, one ply at a time, with intermediate compaction steps taken after every application of a few plies. This is very time-consuming and costly. Also the external shape design of such club heads is restricted to shapes which are more rounded in nature so the female tool applies a more uniform pressure to the exterior of the part. This imposes fairly serious shape design constraints. Most high performance club heads do not have these rounded shapes.
The club heads produced by the present invention do not suffer from these limitations. The high pressure exerted by the internal pressure bladder produces a very high quality product with low void content, high fiber volume laminate, better than the best costly compression molded club heads. The heads are also hollow which is a major benefit. Since the high compaction pressures are produced by the interior bladder, and not dependent on extremely precise sizing and placement of the uncured fiber resin material, the time and cost associated with laying up the uncured fiber resin material is reduced substantially. Also, since the compaction pressures are not dependent on the sizing and interaction of foam cores, the uncured fiber resin material, and the inside of the female tool, the club heads of the present invention have essentially no limitations in their shape.
A club head used with a particular shaft in a particular golf club type (i.e., a "driver" which is also known as a "number one wood", "number two wood", etc.) will have an optimum weight chosen to maximize the playability of the club. Generally this can be stated as a particular club type having a given shaft length will need to have a predetermined set of mass properties to obtain an optimal configuration for playability and performance. These mass properties are the balance point and the swing weight (the center of gravity and mass moments of inertia). Therefore a given club head will have a set weight in its optimal design, not the lowest weight possible. Typically, for example, a "driver" ("number 1 wood") will have a club head weight of about 200 grams.
Generally, for most golfers using a given golf club type, larger club heads are more desirable and work better than smaller club heads, especially for wood-type heads. This benefit can be understood in the simplest sense by realizing that it is easier to hit the ball (not miss hitting it) with a larger club head. More precisely, it can be understood that a larger club head will generally have larger mass moments of inertia, and thus will be more stable and less prone to rotate on off-center hits during ball impact. Sometimes golf club heads are said to have "sweet spots" on the club head face. These areas are dependent on the mass properties of the club head, and also on the elastic properties of face. In general, the larger the size of face of the club, the larger the "sweet spot," which makes it easier for a golfer to strike the ball.
Given these design constraints on the club head, intelligently designed composite club heads do not use composite materials to decrease the club head weight, but to decrease the amount of material needed to provide the necessary strength and stiffness in the head. This has been termed herein "reducing the minimum structural weight."
Once the minimum amount of composite material needed for strength and stiffness has been obtained, additional material is then added to the head, either composite material or metallic material, to bring the head weight up to the desired optimal weight. Reducing the minimum structural weight maximizes the flexibility for location of this additional weight. The placement and location of the weight inside the club head have a very pronounced effect on the playability and performance of the club head. As mentioned, the final mass properties of the weighted club head, primarily the center of gravity and mass moments of inertia, have a large impact on the overall club performance. Reducing the minimum structural weight maximizes the tailoring which can be accomplished on mass properties, which improves club playability.
As an example, a lightweight titanium head might have 90% of its weight located as needed for structural performance. That means that 20 grams out of the 200 gram weight might be available to be placed where needed for mass properties. An equivalent composite carbon fiber epoxy head of the present invention would have 65% of its weight located as needed for structural performance, allowing 70 grams to be placed where needed for optimal mass properties.
As mentioned, larger club head size generally improves playability. Club head size is usually termed and measured as club head volume. The lower density of composite materials also allows for the manufacture of larger club heads which have better playability. The higher strength of club heads of the present invention also allows even larger club head volumes than previously offered by lower strength composite club heads. As explained in the detailed description of this invention, the method of the present invention produces high quality composite heads, in part because there is no one location in the head where several ply terminations occur simultaneously.
The construction methods disclosed herein produce very high performance composite club heads. Further refinement of all the various manufacturing steps for use in high volume production is anticipated. It is expected that it will be recognized by those skilled in the art that the many variations possible without departing from the scope and spirit of this invention.